The present invention relates to a heterojunction bipolar transistor (hereinafter, referred to as HBT) and a manufacturing method therefor.
Together with the functional improvement of digital portable telephones, there has been a demand for even higher performance of transmission-use high-power amplifiers. The HBT is regarded as promising in the field of high-frequency devices. For further higher performance of the HBT, it is necessary to reduce the parasitic device effects, i.e., parasitic resistance and parasitic capacitance. The parasitic resistance can be classified roughly into emitter resistance, base resistance and collector resistance. In particular, the emitter resistance tends to increase together with decreasing emitter dimensions intended for higher performance. This leads to noticeable deterioration of current amplification rate and cutoff frequency of the HBT.
Conventionally, GaAs based HBTs have been provided, where in the case of npn type, the n-type emitter ohmic electrode is provided by using AuGe based metals, W or other high melting point metals, WN or other high melting point nitrides and WSi or other high melting point silicides, the p-type base ohmic electrode is provided by using Pt, Pd, AuZn, AuBe or the like, and the n-type collector ohmic electrode is provided by using AuGe based metals. Examples of such an HBT and an ohmic electrode used therefor are shown in the following figures (1)-(3):
(1) FIG. 14 is a schematic sectional, structural view of main part of an HBT disclosed in Reference, S. Hongo et al., SSDM, 1994, pp. 613-615. In FIG. 14, a semi-insulating substrate and an n-type collector contact layer formed on the semi-insulating substrate and having a high carrier concentration are omitted.
On the collector contact layer, as shown in FIG. 14, a low-concentration n-type collector layer 40, a high-concentration p-type base layer 41, a low-concentration n-type emitter layers 42, 43, and a high-concentration n-type emitter contact layer 44 are formed one by one. Further, on the high-concentration n-type emitter contact layer 44 is formed an emitter ohmic electrode 46. Also, on base protective layers 45 which are both end portions of the low-concentration n-type emitter layer 42, are formed base ohmic electrodes 47, 47. Alloyed reaction layers 48, 48 are formed between these base ohmic electrodes 47, 47 and the high-concentration p-type base layer 41, and device isolating layers 49, 49 are formed so as to sandwich the base layer 41 and the emitter layer 42. The alloyed reaction layers 48, 48 extend through the base protective layers 45, 45 to connect the base ohmic electrodes 47, 47 and the base layer 41 to each other.
The base protective layers 45, 45 formed so as to cover the base layer except its portion placed just under the emitter layer 43 (hereinafter, referred to as external base) out of the base layer 41 are provided in order to suppress decreases of the current amplification rate. That is, when the surface of the external base out of the high-concentration p-type base layer 41 is not covered by the base protective layers 45, a multiplicity of interfacial levels present at the surface causes carrier recombination to occur at the surface of the external base out of the base layer 41, incurring a deterioration of the current amplification rate. However, by the base protective layers 45 covering the surface of the external base out of the base layer 41, these base protective layers 45 are completely depleted during the device operation, so that the supply of carriers to the base protective layers 45 is cut off. As a result, the recombination of carriers is suppressed at the high-concentration p-type base layer 41 so that the deterioration of the current amplification rate can be suppressed.
However, when the base ohmic electrodes 47 of the high-concentration p-type base layer 41 are formed, the surface of the external base out of the base layer 41 is covered with the base protective layers 45, keeping the high-concentration p-type base layer 41 and the ohmic electrodes 47 from making contact with each other, with the result that an ohmic contact cannot be obtained. Therefore, Pt alloyed reaction layers 48, 48 are formed by forming Pt/Ti/Pt/Au one by one and then subjecting these to heat treatment, and further the high-concentration p-type base layer 41 and the base ohmic electrodes 47, 47 are put into contact with each other by the Pt alloyed reaction layers 48, 48, by which an ohmic contact is obtained. In this case, the Pt alloyed reaction layers 48, 48 are required to extend through the base protective layers 45 and reach the high-concentration p-type base layer 41. Also, the emitter ohmic electrode 46 is formed of Ti/Pt/Au by a process other than the process for the base ohmic electrodes 47, 47. Although not shown, the collector ohmic electrode is a AuGe based electrode.
(2) FIG. 15 is a schematic sectional view of main part of an HBT disclosed in Reference, E Zanoni et al., IEEE Device Letters, Vol. 13, No. 5, May 1992. In this HBT, as shown in FIG. 15, on a semi-insulating substrate 50 are formed an n-type GaAs collector contact layer 51, an n-type GaAs collector layer 52, a p-type GaAs base layer 53, a non-doped GaAs layer 54, an n-type AlGaAs emitter layers 55, 56, 57, an n-type GaAs emitter contact layer 58, an n-type InGaAs emitter contact layers 59, 60, one by one. Also, a base ohmic electrode 61 for the p-type base layer 53 is formed of AuBe, an emitter ohmic electrode 62 for the n-type emitter layers 55, 56, 57, as well as a collector ohmic electrode 63 for the n-type collector layer 52 are formed of AuGeNi. Accordingly, the emitter ohmic electrode 62 and the collector ohmic electrode 63 can be formed simultaneously. That is, the emitter ohmic electrode 62 and the collector ohmic electrode 63 can be formed by one manufacturing step.
(3) FIGS. 16A, 16B and 16C are schematic sectional views of an ohmic electrode disclosed in Japanese Patent Laid-Open Publication HEI 8-222526. As shown in FIG. 16A, a Cu layer 71, a Ge layer 72 and Cu layer 73 are stacked one by one on a high-concentration n- or p-type GaAs layer 70, and an ohmic electrode is obtained by annealing these layers. Also, as shown in FIG. 16B, a Pd layer 75, a Cu layer 76, a Ge layer 77 and a Cu layer 78 are stacked one by one on a high-concentration n- or p-type layer 74, and an ohmic electrode is obtained by annealing these layers. Further, as shown in FIG. 16C, a Cu layer 80, a Pd layer 81, a Ge layer 82 and a Cu layer 83 are stacked one by one on a high-concentration n- or p-type GaAs layer 79, and an ohmic electrode is obtained by annealing these layers. The ohmic electrodes of FIGS. 16A, 16B and 16C can be used as ohmic portions for the high-concentration p-type GaAs layers and the high-concentration n-type GaAs layers in the HBTs. Therefore, ohmic portions for the high-concentration p-type GaAs layers and the high-concentration n-type GaAs layers can be formed by one manufacturing step, thereby facilitating the formation process for the ohmic portions.
However, the HBTs as described in (1), (2) and (3) and the ohmic electrodes to be used therefor have the following problems.
With regard to (1), the process for forming the emitter ohmic electrode 46, the base ohmic electrodes 47 and the collector ohmic electrode includes a resist formation step for forming photoresist of a pattern corresponding to the configuration of the ohmic electrodes, a metal thin film formation step for forming a metal thin film by using vapor deposition or sputtering process, and a so-called lift-off step for leaving the metal thin film only at necessary portions by removing the photoresist. Also, the base ohmic electrodes 47 are formed of Pt/Ti/Pt/Au, the emitter ohmic electrode 46 is formed of Ti/Pt/Au, and the collector ohmic electrode is formed of AuGe. Therefore, the emitter ohmic electrode 46, the base ohmic electrodes 47 and the collector ohmic electrode cannot be formed simultaneously. As a result, the emitter ohmic electrode 46, the base ohmic electrodes 47 and the collector ohmic electrode need to be manufactured by different processes, making it necessary to perform the resist formation step, the metal thin film formation step and the lift-off step for each of the emitter ohmic electrode 46, the base ohmic electrodes 47 and the collector ohmic electrode. This leads to a problem that the manufacturing cost would increase with increasing number of manufacturing steps. Moreover, use of expensive materials such as Pt and Au causes the manufacturing cost to further increase.
With regard to (2), the emitter ohmic electrode 62 and the collector ohmic electrode 63, which are formed of AuGeNi, can be formed by one manufacturing step at the same time. However, in such Au-based ohmic electrodes as the AuGeNi-based one, performing heat treatment may cause the electrode metals to react nonuniformly and flocculate into island-like shapes, resulting in nonuniform ohmic contacts within the electrode regions. In such a case, the ohmic contact resistance would not lower enough, and the emitter resistance and the collector resistance would increase. This leads to a problem that the current amplification rate and the cutoff frequency are deteriorated, in particular, because of increase in the emitter resistance. The emitter resistance tends to further increase by scale-down of the emitter size intended for higher performance.
With regard to (3), the contact resistivity of ohmic electrodes after annealing is about 1xc3x9710xe2x88x926 xcexa9cm2 for p-type GaAs, but it is 1xc3x9710xe2x88x925 xcexa9cm2, one digit higher, for n-type GaAs. Therefore, when these ohmic electrodes are used for an npn-type HBT, the emitter resistance especially of n type increases, leading to a problem that the current amplification rate and the cutoff frequency are deteriorated, as in the case of (2).
The cutoff frequency fT of an HBT can be expressed as
fT={2xcfx80(xcfx84E+xcfx84B+xcfx84C+(RE+RC)Cbc)}xe2x88x921 xe2x80x83xe2x80x83Eq. (1) 
where, in Equation 1, xcfx84E is the time constant of the emitter, xcfx84B is the base running time, xcfx84C is the collector running time, RE is the emitter resistance, RC is the collector resistance, and Cbc is the base-collector capacity.
It can be understood from Equation (1) that decreases of the emitter resistance RE and the collector resistance RC are effective for improvement of the cutoff frequency fT. A decrease of the collector resistance RC can be obtained by making the collector part sufficiently thick to lower the sheet resistance of the collector. Therefore, it is the emitter resistance RE that demands special consideration. Since the emitter size will be further scaled down for higher performance in the future, a contact resistivity of 1xc3x9710xe2x88x926 xcexa9cm2 or less is essential also for n-type GaAs.
It is therefore an object of the present invention to provide an HBT, as well as a manufacturing method therefor, which allows the manufacturing cost to be decreased and which shows a successful contact characteristic.
In order to achieve the above object, there is provided a heterojunction bipolar transistor comprising: a low-concentration n-type collector layer formed a semi-insulating substrate; a high-concentration p-type base layer formed on the collector layer; a low-concentration n-type emitter layer formed on the base layer; a base ohmic electrode which is made of a single layer or a plurality of layers and which is formed on a base protective layer that is a portion of the emitter layer; a high-concentration n-type emitter contact layer formed so as to cover regions of the emitter layer except the base protective layer; and an emitter ohmic electrode which is made of a single layer or a plurality of layers and which is formed on the emitter contact layer, further comprising:
a base-use alloyed reaction layer formed under the base ohmic electrode, and an emitter-use alloyed reaction layer formed under the emitter ohmic electrode, wherein
the base-use alloyed reaction layer extends through the base protective layer so as to reach the base layer and the emitter-use alloyed reaction layer is formed only within the emitter contact layer.
In the HBT of this construction, the base ohmic electrode is connected to the high-concentration p-type base layer through the base-use alloyed reaction layer, and the emitter ohmic electrode is connected to the higher-concentration n-type emitter contact layer through the emitter-use alloyed reaction layer. In this case, the base-use alloyed reaction layer extends through the base protective layer, reaching the base layer, while the emitter-use alloyed reaction layer is formed only within the emitter contact layer, not extending through the emitter contact layer.
Thus, by virtue of the arrangements that the emitter-use alloyed reaction layer is formed only within the higher-concentration n-type emitter contact layer and that the base-use alloyed reaction layer extends through the base protective layer to reach the high-concentration p-type base layer, the barrier width of the junction between the high-concentration n-type emitter contact layer and the emitter-use alloyed reaction layer becomes narrow and the barrier width of the junction between the high-concentration p-type base layer and the base-use alloyed reaction layer becomes narrow, allowing the carriers to pass freely through the barriers by the tunneling effect. Accordingly, the base ohmic electrode and the emitter ohmic electrode show successful ohmic characteristics so that the emitter resistance is decreased, making is possible to improve the current amplification rate and the cutoff frequency.
In one embodiment of the present invention, the base ohmic electrode and the emitter ohmic electrode are made of an identical material.
In the HBT of this one embodiment, since the base ohmic electrode and the emitter ohmic electrode are made of an identical material, the base ohmic electrode and the emitter ohmic electrode can be manufactured by one manufacturing step. Accordingly, the number of manufacturing steps is decreased, so that the manufacturing cost can be reduced.
In one embodiment of the present invention, the emitter contact layer is composed of a first emitter contact layer and a second emitter contact layer formed on the first emitter contact layer;
carrier concentration of the second emitter contact layer is set so as to be higher than carrier concentration of the first emitter contact layer; and
the emitter-use alloyed reaction layer is formed only within the second emitter contact layer.
In the HBT of this one embodiment, since the emitter-use alloyed reaction layer is formed only within the second emitter contact layer having a carrier concentration even higher than the carrier concentration of the first emitter contact layer, the barrier width of the junction between the higher-concentration second emitter contact layer and the emitter-use alloyed reaction layer becomes even narrower, so that the emitter ohmic electrode shows more successful ohmic characteristics. Accordingly, the resistance of the emitter ohmic electrode, i.e., the emitter resistance is decreased, making is possible to improve the current amplification rate and the cutoff frequency.
In one embodiment of the present invention, the collector layer and the base layer are formed of GaAs and the emitter layer and the base protective layer are formed of AlGaAs.
In the HBT of this one embodiment, the arrangement that the collector layer and the base layer are formed of GaAs while the emitter layer and the base protective layer are formed of AlGaAs is desirable for improvement of the current amplification rate and the cutoff frequency.
In one embodiment of the present invention, the first emitter contact layer is formed of GaAs and the second emitter contact layer is formed of InGaAs.
In the HBT of this one embodiment, the arrangement that the first emitter contact layer is formed of GaAs while the second emitter contact layer is formed of InGaAs is desirable for improvement of the current amplification rate and the cutoff frequency.
In one embodiment of the present invention, the base ohmic electrode and the emitter ohmic electrode, or a lowermost layer of the base ohmic electrode and a lowermost layer of the emitter ohmic electrode are made of Pt; and
the base-use alloyed reaction layer and the emitter-use alloyed reaction layer contain Pt.
In the HBT of this one embodiment, the base ohmic electrode and the emitter ohmic electrode, or the lowermost layer of the base ohmic electrode and the lowermost layer of the emitter ohmic electrode are made of Pt, while the base-use alloyed reaction layer and the emitter-use alloyed reaction layer contain Pt. This is desirable for improvement of the current amplification rate and the cutoff frequency.
Also, there is provided a method for manufacturing the heterojunction bipolar transistor as defined in claim 3, comprising the steps of:
stacking, on the base protective layer and the second emitter contact layer, an electrode material that makes the base ohmic electrode and the emitter ohmic electrode, or an electrode material Pt that makes a lowermost layer of the base ohmic electrode and a lowermost layer of the emitter ohmic electrode, so that a film thickness of the electrode material becomes thinner than a film thickness of the second emitter contact layer; and
subjecting the electrode material to a heat treatment to form the base ohmic electrode on the base protective layer and form the emitter ohmic electrode on the second emitter contact layer.
In the HBT manufacturing method of this constitution, the electrode material is stacked on the emitter contact layer and the base protective layer so as to be thinner than the film thickness of the second emitter contact layer. Thereafter, the electrode material is subjected to a heat treatment, by which a base ohmic electrode is formed on the base protective layer and an emitter ohmic electrode is formed on the second emitter contact layer. In this process, a base-use alloyed reaction layer is formed under the base ohmic electrode, and an emitter-use alloyed reaction layer is formed under the emitter ohmic electrode. The base-use alloyed reaction layer extends through the base protective layer, reaching the high-concentration p-type base layer, while the emitter-use alloyed reaction layer is formed only within the high-concentration n-type emitter contact layer. Thus, by making the film thickness of the electrode material thinner than the film thickness of the second emitter contact layer, it becomes possible to make the base-use alloyed reaction layer extend up to the high-concentration p-type base layer and besides to form the emitter-use alloyed reaction layer only within the high-concentration n-type emitter contact layer. As a result of this, the base ohmic electrode and the emitter ohmic electrode can be improved in contact.
In one embodiment of the present invention, the film thickness of the second emitter contact layer is set to three or more times the film thickness of the electrode material Pt.
With the HBT manufacturing method of this one embodiment, it has been found out that by setting the film thickness of the second emitter contact layer to three or more times the film thickness of the electrode material, the emitter-use alloyed reaction layer to be formed by subjecting the electrode material to the heat treatment can be prevented from extending through the second emitter contact layer.
In one embodiment of the present invention, the film thickness of the electrode material is within a range defined by a following relational expression:
(film thickness of the base protective layerxc3x97xc2xd) less than film thickness of the electrode material before heat treatment less than {(film thickness of the base protective layer+film thickness of the base layer)xc3x97⅓}.
With this HBT manufacturing method of this one embodiment, it has been found out that by setting the film thickness of the electrode material to within such a range that the above relational expression is satisfied, the contact resistance of the base ohmic electrode can be decreased and besides that the base-use alloyed reaction layer to be formed by the heat treatment can be prevented from extending through the base layer.
It has also been found out that when the film thickness of the electrode material before the heat treatment is less than xc2xd of the film thickness of the base protective layer, the contact resistance of the base ohmic electrode becomes higher.
Further, it has been found out that when the film thickness of the electrode material before the heat treatment is thicker than ⅓ of {film thickness of the base protective layer+film thickness of the base layer}, the base-use alloyed reaction layer to be formed by the heat treatment extends through the base layer so as to reach the collector layer.